1. Field of the Invention
The present invention relates to semi-hard magnetic materials, which are suited for reed pieces of a self-holding type reed switch, as well as a process for producing the semi-hard magnetic materials.
2. Description of the Prior Art
Such materials should possess a hysteresis loop shape and a residual magnetization property suited for application in a self-holding type reed switch. Specifically, the semi-hard magnetic materials utilized in a reed switch should preferably possess the following properties.
1. Both saturated magnetic flux density (Bs) and residual magnetic flux density (Br) are high.
2. The hysteresis loop exhibits a high squareness ratio (Br/Bs) and a high "fullness factor". The term "fullness factor" used herein is represented by the formula ##EQU1## wherein (BH)max is the maximum magnetic energy product and Hc is the coercive force.
3. The coercive force (Hc of magnetic materials) is usually in the range between 10 and 50 Oe, but the coercive force should be more than 50 Oe for application of semi-hard magnetic materials to reed switches where mutual demagnetization, explained hereinafter, is severe.
4. Plastic workability is excellent, i.e. these materials are capable of being easily cold worked into any desired shape or size such as, for example, a fine wire rod having a diameter as small as less than 1 mm.
5. The plating or soldering property is excellent, i.e. a surface layer on the semi-hard magnetic materials used for forming a reed switch can be reliably produced by a plating or soldering process.
6. When the semi-hard magnetic materials are exposed to a high temperature, at the time of sealing the reed pieces having the above mentioned surface layer in a glass capsule of the reed switch, the magnetic properties of the reed pieces are not deteriorated by the increase of temperature.
When the reed switches are used in a four-wire crosspoint switch matrix of an electronic switching system, mutual demagnetization occurs between the four reed switches. As a result of the demagnetization, the coercive force and the squareness ratio of the semi-hard magentic materials must be higher than in the reed-switches used in a two-wire crosspoint switch of an electronic switching system.
In U.S. Pat. No. 3,989,557, filed by the present inventors, there is described a process for producing a semi-hard magnetic material, wherein an alloy material comprising from 73 to 93% of cobalt, from 1 to 5% of niobium and iron in balance is subjected to (1) a process annealing at a temperature of 900.degree. or higher, and subsequently, (2) a final cold working at a reduction of area of 75% or higher. The same U.S. patent also describes a process for producing a semi-hard magnetic material, wherein the alloy material, which is cold worked as stated above, is further subjected to aging at a temperature of 600.degree. to 900.degree. C. and higher. The semi-hard magnetic material, in the U.S. patent mentioned above, however, is disadvantageous in the fact that the cold working property thereof is deteriorated by the increase of the niobium content in the range of from 1 to 5%, although the coercive force of the semi-hard material is increased by such increase of niobium. In addition, deterioration of cold-workability is particularly remarkable when an attempt is made to increase niobium to more than 5%.
The alloy material of the U.S. Patent mentioned above may contain, for example, tantalum, titanium, vanadium, zirconium, molybdenum, chromium, tungsten and the like. The amount of these metals used in addition to niobium is usually such that the ratio by weight of these metals to niobium is less than 30/70. These metals form intermetallic compounds in the same manner as niobium forms the intermetallic compounds, according to the description in the above U.S. Pat. No. 3,989,557.
An improved process for preventing the deterioration of the cold workability is proposed in U.S. Pat. No. 3,983,916 assigned to the same assignee as the present invention, wherein an alloy material comprising essentially from 73 to 93% of cobalt, from 3 to 7% of niobium and iron in balance is subjected to (1) a solution treatment, i.e. a heating at a temperature of 1000.degree. C. or higher followed by a rapid cooling through at least the temperature range between 800.degree. C. and 500.degree. C., then, (2) a cold working at a reduction area of 75% or more, and subsequently, (3) an aging at a temperature in the range from 500.degree. to 900.degree. C. The present inventors investigated further in detail the improved process mentioned above and realized that this process involves the following problem, although the cold workability of the semi-hard magnetic material is in fact enhanced to some extent by the improved process. First, unless the bar or wire of the semi-hard magnetic is carefully drawn in the cold drawing process, the bar or wire is liable to rupture. Second, the cold workability of the semi-hard magnetic material is seriously degraded, to such an extent that the cold working cannot be practiced industrially, when an attempt is made to provide the material with a coercive force higher than 40 Oe. In the semi-hard magnetic material mentioned above as containing 3 to 7% of niobium, the Nb content and Fe content must be increased to 5% or higher and 18% or higher, respectively, to produce the result that the coercive force is increased more than 40 Oe, but the cold workability is deteriorated. It is, therefore, practically impossible to obtain from the Co-Fe-Nb alloy material, a semi-hard magnetic material having a coercive force of from 40 to 70 Oe and which is suited for the reed switches to be mounted in a four-wire crosspoint switch matrix of an electronic switching system.
A semi-hard magnetic material having a high coercive force is available under the name of VACOZET 655, which is sold by the VACUUMSCHMELZE Company, in the Federal Republic of Germany. This semi-hard magnetic material comprises 30% iron 10.6% nickel, 1.2% aluminum, 2.4% titanium and cobalt in balance. Since the magnetic properties of this semi-hard magnetic material are unstable, with the result that, for example, the coercive force is reduced by heat, the material is not suitable for reed pieces, which are sealed or enclosed in a glass capsule of a reed switch. In addition, since the semi-hard magnetic material of VACOZET 655 includes aluminum as well as titanium, the plating and soldering properties of the material are not superior to those of the Co-Fe-Nb alloy.